Surface mounting has become the preferred technique for circuit board assembly and virtually all types of electronic components have been or are being redesigned for surface mount, that is, leadless, applications. The rapid incorporation of surface mount devices (SMD) into all types of electronic circuits has created a demand for SMD fuses.
Fuses serve an essential function on many circuit boards. By fusing selected sub-circuits and even certain individual components it is possible to prevent damage to an entire system which may result from failure of a local component. For example, fire damage to a mainframe computer can result from the failure of a tantalum capacitor; a short in a single line card might disable an entire telephone exchange.
The required characteristics for circuit board fuses are small size, low cost, accurate current-sensing, very fast reaction or blow time and the ability, in the case of time lag fuses, to provide surge resistance.
Prior art tube type or leaded fuses take up excessive space on circuit boards designed for SMD assembly and add significantly to production costs. Recognizing the need for fuses compatible with SMD assembly techniques, several manufacturers offer leadless, molded fuses for standard SMD assembly. The devices provided by this approach, however, remain bulky (for example, package sizes of about 7.times.4.times.3 mm), expensive and of limited performance range. Most importantly, the characteristics of fuses of the aforedescribed prior art cannot be accurately controlled during manufacture.
The above referenced U.S. patent and application show that thin film technology provides a high level of control of all fuse parameters, thus making possible economical standard and custom fuse designs meeting a wide range of fusing requirements. Thus, thin film technology enables the development of fuses in which both electrical and physical properties can be tightly controlled. The advantages of the technology are particularly evident in the areas of physical design, repeatability of fusing characteristics and I.sup.2 t "let-through". Moreover, because present techniques allow line width resolution below 1 .mu.m and control of layer thickness to 100 .ANG., the fabrication of true miniature SMD fuses having standard (for example, 1.6.times.0.8 mm) and non-standard package sizes are made possible.
The referenced U.S. patent and application also disclose methods of manufacturing a thin film surface mount electrical fuse in which, first, a uniform thin metal film of aluminum is deposited by sputtering or the like on a surface of an insulating substrate such as glass. The thickness of the film is dependent upon, among other things, the fuse rating. Selected portions of the thin metal film are then removed by photolithographic techniques to define a repetitive pattern comprising a plurality of identical fuse elements each comprising a pair of contact portions interconnected by a fusible link having a width smaller than that of the contact portions. The structure is then passivated and an insulating cover plate of glass is bonded by epoxy over the passivation layer. The assembly formed by the preceding steps is next cut into strips along end planes normal to the surface of the substrate, each strip including a series of side-by-side fuses. This cutting step exposes edges of the contact portions of each fuse element along the end planes of the strips. Conductive termination layers are deposited over the end planes thereby electrically connecting the terminations to the exposed edges of the contact portions. Last, the strips are cut transversely into individual fuses.
The photolithographic production method allows a great variety of fuse element designs and substrate types to be combined for creating a wide range of fuse chips. Moreover, critical parameters such as fuse speed can be programmed to optimally satisfy application requirements. Finally, the hermetic structure of the thin film fuse provided by the sealing glass cover plate imparts excellent environmental reliability.
The glass-fuse-glass layered structure described in the aforementioned patent application does have several limitations. Although glass has the desirable thermal properties needed for "quick" rated thin film fuses, it is fragile. The voltage ratings of such fuses are thus limited, for example, to 32 volts, because the application of high voltages would fracture the fuse body. The mechanical bending strength of the glass-fuse-glass structure is likewise limited by the fragility of the glass, as is the thermal cycling ability of the structure.
Accordingly, it is an overall object of the present invention to provide a thin film surface mount fuse having voltage ratings that are higher than previously possible while maintaining the "quick" rating thereof.
It is another object of the invention to provide a thin film surface mount fuse structure that has improved mechanical strength and reliability, as well as a greater thermal cycling ability.
It is yet another object of the present invention to provide methods of manufacturing such improved thin film surface mount fuses.